Anatase titanium dioxide (TiO2) has been widely studied during recent decades for photocatalytic applications such as water purification, water splitting and solar cells due to its low cost, high chemical stability and excellent charge transport ability. However, because of its large band gap energy (≈3.2 eV), it shows a poor photocatalytic activity by visible light requiring UV light activation. It has been shown that doping TiO2 with transition metal ions can improve its photocatalytic activity under visible light irradiation. It is believed that the interaction of the 3d orbital of Ti and d orbital of the transition metal introduces an intra-band gap state causing a decrease in the band gap energy, which leads to a red shift (longer wavelengths) in absorption of the photon. Furthermore, metal dopants inhibit electron/hole recombination that enhances the photocatalytic activity, due to the effective separation of the charge carriers. The fast recombination of photo generated electrons and holes reduces the photo catalytic activity of TiO2. Using metal dopants, electrons and/or holes can be trapped, leading to the increased life time of the charge carriers enhancing their time to reach the catalyst's surface for initiation of the photo catalytic reactions. It has been reported that among transition metals, Fe3+ can inhibit electron/hole recombination the best by trapping the photo generated electrons and holes. Since the energy level of Fe3+/Fe4+ is above the valence band energy and Fe3+/Fe2+ is below the conduction band energy of TiO2, Fe3+ can react with either a hole or an electron forming Fe4+ or Fe2+ trap reducing the recombination of charge carriers. On the other hand, Fe2+ and Fe4+ are less stable than Fe3+ based upon the crystal field theory involved with gaining or losing an electron, causing them to eventually revert back to the Fe3+ state, which would release the electron and hole for their migration to the surface of the catalyst to initiate the photo catalytic reactions.
A previous report using XPS and atomic absorption spectroscopy indicated that the surface concentration of Fe3+ is significantly higher than the bulk concentration (Bajnóczi, É. G.; Balázs, N.; Mogyorósi, K.; Srankó, D. F.; Pap, Z.; Ambrus, Z.; Canton, S. E.; Norén, K.; Kuzmann, E.; Vértes, A.; Homonnay, Z.; Oszkó, A.; Pálinkó, I.; Sipos, P., The influence of the local structure of Fe(III) on the photocatalytic activity of doped TiO2 photo-catalysts—An EXAFS, XPS and Mössbauer spectroscopic study. Applied Catalysis B: Environmental 2011, 103, 232-239).
The role of Fe3+ is still under debate; Serpone et al. (Spectroscopic, Photoconductivity, and Photocatalytic Studies of TiO2 Colloids: Naked and with the Lattice Doped with Cr3+, Fe3+, and V5+ Cations. Langmuir 1994, 10, 643-652) reported that iron increases the recombination of electrons and holes, which is detrimental to the photocatalytic activity, where they described that by increasing the amount of dopant, the photocatalytic activity decreased. On the other hand, Choi et al. (The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. The Journal of Physical Chemistry 1994, 98, 13669-13679) and Zhou et al. (Zhou, M.; Yu, J.; Cheng, B.; Yu, H., Preparation and photocatalytic activity of Fe-doped mesoporous titanium dioxide nanocrystalline photo-catalysts. Materials Chemistry and Physics 2005, 93, 159-163) suggested that adding iron ions as the dopant decreases electron/hole recombination and increases the photocatalytic activity. They concluded that Fe3+ can trap both electrons and holes, which is favorable for photocatalytic efficiency. However, based upon the previous studies, the degradation efficiency of Fe-doped TiO2 is low (7.8% and 5.5%) degradation of methyl orange within 5 and 3 hours of reaction time, respectively) under visible light illumination (Kerkez-Kuyumcu, Ö.; Kibar, E.; Dayioğlu, K.; Gedik, F.; Akin, A. N.; Özkara-Aydinoğlu, Ş., A comparative study for removal of different dyes over M/TiO2 (M=Cu, Ni, Co, Fe, Mn and Cr) photo-catalysts under visible light irradiation. Journal of Photochemistry and Photobiology A: Chemistry 2015, 311, 176-185). Thus, doping with iron may or may not allow ultraviolet light to be replaced with visible light and if it can be, the efficiency of breakdown is extremely low.
What is needed is a method of increasing the photocatalytic activity of Fe-doped titanium dioxide. It would be advantageous if the method was low cost and easy to perform. It would be advantageous if the method provided a significantly better product. It would be of further advantage if the product was used in a method for cleaning of waste water and other contaminated aqueous solutions using visible light. More specifically, it would be advantageous if the product and method of use could be used to treat ammonia in waste water as there is no effective way to treat this ammonia. It would also be advantageous if the product and the method of use could be used to treat organics in waste water. The organics in waste water are now treated by biological means that are susceptible to infections, temperature, pH, etc. and poisoned by ammonia requiring the waste water to be diluted by fresh water or sea water that further impacts our environment. It would be of further advantage if the product and method could control, inhibit or eliminate microbial growth.